1. Field of the Invention
This invention relates to a developing unit used in a copying machine, a laser printer, etc. for developing, for example, an electrostatic latent image formed on a recording medium.
2. Description of the Prior Art
In a copying machine or the like, a developing unit is provided for making visible an electrostatic latent image formed on a recording medium such as a photoconductor by making toner, which is made from color pigment, adhere to the electrostatic latent image. Furthermore, some copying machines or the like are provided with a plurality of developing units containing toners of different colors for converting an electrostatic latent image into a color image, not merely into a black and white image, by selectively using the developing units.
Such developing units must be controlled so that toner will not adhere to the latent image when the units are not used for developing the image. For this reason, the developing gap (the gap between the recording medium and the developing section of the developing unit) is switched between different settings to prevent the toner from contacting with or being applied to the developing areas facing the recording medium.
However, changing the developing gap between different settings causes variations in the developing conditions, thus resulting in the variation in the copy image density or other trouble. Especially, if the positioning of the developing gap is not made precisely, it is not possible to invariably develop a normal image.
Some developing units employing a magnetic brush developing method are controlled in such a way that the developing brush does not contact the recording medium by shifting the positions of the magnets that form the developing section. However, failure to properly shift the positions of the magnets will not only cause variations in the copy image density but also lead to substantial deterioration of the copy quality because of the adherence of carrier to the recording medium (in the case of two-component developer).
Accordingly, the applicant of the present invention proposed a technique such as disclosed in Japan Patent Application No. 62-222661. This technique uses a developing roller disposed in a location facing a recording medium, a supply passage for supplying developer to the developing roller, a shut-off device for opening and closing the supply passage, and a switching means having a spring clutch for switching the shut-off device between the open and closed states, and offers the advantage that because the supply passage is opened and closed by operating the blocking plate of the shut-off device, the gap between the developing section and the recording medium is always maintained constant, and therefore, the developing conditions remain unchanged, thus assuring invariable production of normal copy images.
In the above construction, however, because the input and output sides of the spring clutch are fixed, it presents the problem that when the shut-off device is in a stop position, if the center axis of the shut-off device is not aligned with the center shaft of the spring clutch, rotational driving force is transmitted from the input side to the output side of the spring clutch because of the spring force within the clutch, thereby changing the stopping position of the shut-off device for opening or closing operation.